Continued global emphasis and government legislation on reducing emissions and improving fuel economy of internal combustion engines has led to the need to develop advanced high efficiency, clean combustion engines. Exhaust after-treatment systems (such as Selective Catalyst Reduction (SCR), lean NOx traps, and diesel particulate filters) have been designed and commercialized to lower exhaust emissions to meet emission targets and regulations. However, these systems are costly, add to the weight of the vehicle, and minimize fuel economy due to the added weight and the need to use fuel to regenerate the systems. Reducing engine-out emissions would decrease the size and/or eliminate the need for these systems. Worldwide, a large R&D effort is underway at a multitude of industrial, government, and academic organizations to identify engine designs, operating conditions, and fuel compositions to accomplish that objective. One advanced combustion approach being considered is Homogeneous Charge Compression Ignition (HCCI) in which fuel is injected very early into the engine to enable a homogeneous mixture of air and fuel to be obtained prior to the start of combustion initiated through compression ignition. One significant drawback to that approach is that it is difficult to control the combustion process and high pressure rise; and, furthermore, heat release rates occur resulting in unacceptably high noise levels and potential engine damage. Thus, currently the operating speed-load range where acceptable performance can be obtained is very limited.
Another approach to optimize engine designs, operating conditions and fuel compositions is to employ fuels in a Partially Premixed Combustion (PPC) environment. In PPC settings, fuel injection timing is closer to top dead center and so the air and fuel are not completely mixed prior to combustion. By applying this strategy with high rates of cooled exhaust gas recirculation/recycle (EGR), the combustion event occurs and results in low soot and low NOx. As compared to Homogeneous Charge Compression Ignition (HCCI), the control of the combustion in a PPC engine environment is re-gained along with the potential to reduce the rate of heat release and the maximum pressure rise rate.
Partially premixed combustion has been known to potentially reduce NOx and soot for diesel engines. However, to this point, specific fuel compositions have not been developed to obtain the best synergy among the fuel mixture, partially premixed combustion and reduction in NOx and soot.
We have discovered that specific gasoline fuel compositions having research octane numbers from about 69 to about 99 can have high gross efficiencies exceeding 50% and enable operation over a wide load range (up to or exceeding 18 bar gross IMEP) and provides significant reductions in NOx and soot when used in a PPC-type mode in compression ignition engine environment. Further, within the gasoline boiling range, fuel properties and certain fuel composition have been found to significantly influence the pressure rise rate; and engine performance fuel compositions have been found which lead to acceptable values. Best overall performance was achieved with fuels having a sum total n-paraffins plus naphthenes content greater than 7 volume %.